The present invention relates to recording apparatus for recording measured event data along with a time reference for the event data; and more particularly to a recorder of the type used by electrical utilities to record energy consumption by customers. The recorder includes non-volatile memory providing permanent storage for usage (i.e. event) data along with time reference data and which can be removed at the end of a billing period for remote tabulation or load analysis and replaced with an unrecorded memory for the next billing period.
Billing or survey recorders are used by electrical utility companies for recording the event data provided by an electrical meter which indicates the amount of energy used by a consumer. The billing recorder accumulates data for a period of time called a read or billing period, typically a month long, and it also stores time signals or "marks", generated by the billing recorder. A time mark is simply a signal recorded at a predetermined interval (called "demand" periods) during which the associated event data occurred. That is, the distance between time marks on the tape define a predetermined time period, assuming the tape speed is constant and the same for both recording and playback. Demand periods conventionally are 5, 15, 30 or 60 minutes.
The event data and the time marks of conventional recorders are typically recorded on magnetic tape in a cartridge to allow processing or analysis of the data at a central translation center which is remote from the point of service. The recorded data provides time marks and event data pulses for customer billing and load analysis, but it does not provide data representative of a particular start time or the source (recorder) from which it came. Start and stop times, customer identification such as meter or recorder number (I.D.), and beginning and end meter register readings are all recorded in writing by the exchange personnel, thereby leaving considerable room for human error. When such error does occur, any loss must be suffered by the utility, not the customer.
Most billing recorders in use today employ magnetic tape as the storage medium, even though magnetic tape has temperature and humidity limitations which make it less reliable as a storage medium than is desired in the demanding environment of use by a utility. The tape must be advanced past the record head continuously and at a precise speed during recording. Accordingly, a complex mechanical tape drive system is required to insure proper operation at all environmental specifications. The requirement of drive motors for advancing the tape adds considerable cost and limits miniaturization of the unit. Also, periodic servicing is required to maintain the recording mechanism drive elements, battery carryover system, and to periodically clean the recording head.
It will be appreciated that such recorders are required to operate in a wide range of temperature (typically -20.degree. C. to +65.degree. C.), due to the worst cases of heat and cold they are likely to encounter over the large geographical area in which a given model is marketed.
A further consideration affecting cost, reliability and performance is that billing recorders employed for recording data representing electrical energy usage are occasionally subjected to power interruptions. In recent years, there has been a trend to employ power outage circuits which provide transfer to an auxiliary power source, such as a battery during intervals of primary power loss. It is evident that maintaining the drive to the tape advance motors during primary power loss results in a heavy drain on the battery, thereby limiting the carryover time for which recording can be continued. This is particularly disadvantageous in cold climates.